The invention relates to a process for the deposition, on optical substrates, of antireflection coatings capable of being chemically engraved, by evaporation of a metal in a reactive atmosphere within the enclosure of an evaporator under vacuum.
It is frequently necessary, in optical apparatus, to obtain reticules or frames which are opaque in a field in which an image is formed, in order to define a sighting axis, or to measure paraxial angles, for example. Direct engraving, such as diamond or chemical engraving, of an optical component, more especially a glass plate, has proved to be limited as regards accuracy and definition. In order to achieve the required opacity of the lines which is necesary in order to obtain good contrast, it is often required to fill the lines with a pigment. It has been found that this technique does not permit ultimate levels of resolution, which approach the wavelengths of the visible spectrum.
A technique which has been recommended is the deposition of opaque coatings by methods of deposition under vacuum, by evaporation or by cathodic atomisation, the pattern of the frame or reticule being obtained either by subsequent chemical engraving through a mask or micro-photolithography resist, or by carrying out of the depositing operation on a substrate covered by a mask similar to microphotolithographic masks.
U.S. Pat. No. 3808035 describes a method of deposition in the vapour phase, the deposit being formed by reaction, in contact, of the heated substrate formed of an evaporated compound with a reactive gas, which is diluted in a neutral carrier gas under a pressure close to atmospheric pressure. According to this document, the preferred coatings are metallic oxides, the reactive gas then containing a small quantity of oxygen or of a volatile oxidising agent. Compounds capable of evaporation are, inter alia, organo-metallic compounds, acetyl-acetonates for example, metal carbonyls or oxychlorides.
It will be noted that, according to this document, in order to obtain opaque coatings, reduction of the oxide which is deposited is carried out in order to form a reflecting metallic coating, the reduction being performed by adding to the carrier gas a reducing gas, such as hydrogen, carbon monoxide or methane.
It is clear that the reflections of the light on the frames or reticules cause losses of resolution or of contrast.
The use of coatings of a specific thickness, related to the wavelength, permits the elimination or at least the attenuation of the reflections of a metallic coating. In practice, although the deposition of an oxide coating having a controlled thickness on top of a metallic deposit permits the achievement of reflectances below 2% of the specular reflectance of the surface opposite the substrate, the reflectance at the substrate/deposit interface remains relatively high.
The processes for deposition according to the above-cited document are relatively slow (of the order of ten minutes for a single coating) and consume relatively large quantities of reagents and carrier gases.
An attempt has been made to obtain metallic oxide coatings by evaporation of the base metal in an oxidising atmosphere at pressures below 100 millipascals.
Titanium oxide appeared to be promising from this point of view. However, the chemical engraving solutions which are effective in the case of titanium oxide exhibit a significant degree of corrosiveness in relation to silica, which is a component of glass substrates.